1. Technical Field
The present disclosure relates to an envelope-tracking power amplifier, and more particularly, to an apparatus for controlling time delay occurring in an envelope-tracking modulator.
2. Description of Related Art
Recent mobile communication systems are evolving into communication systems based on Orthogonal Frequency-Division Multiplexing (OFDM) with a large amplitude variation, which is different from the past communication systems such as Global System for Mobile Communications (GSM). For example, Worldwide Interoperability for Microwave Access (WiMAX) and Wireless Broadband Internet (WiBro), as well as the long-term evolution (LTE) which is currently in the spotlight, use OFDM-based communication methods.
However, the OFDM-based transmitter has a disadvantage in that its power amplifier may not operate efficiently due to high peak to average power ratio (PAPR) characteristics. In order to overcome the above drawback, various transceiving techniques have been proposed, one of which is an envelope-tracking (ET) power amplifier.
In the ET power amplifier, a signal having a waveform similar to the envelope of the power amplifier input signal is applied to a power supply unit of the power amplifier so that the power amplifier always operates in the saturation region, thereby achieving high-efficiency performance.
However, in case that the envelope signal applied to the power supply unit of the ET power amplifier and the communication signal output after power amplification in the power amplifier are not well time-synchronized, nonlinearity in the output of the power amplifier may be greatly increased, and Adjacent Channel Leakage Ratio (ACLR) may also be increased so that it may be difficult to transmit signals wirelessly.
One of the biggest obstacles to synchronization between the power amplifier output signal and the envelope signal output from an ET modulator is that the ET modulator has a large time delay due to the relatively narrow bandwidth. Therefore, minimizing the time delay of the ET modulator is an effective way to synchronize the output signal of the ET power amplifier and the envelope signal output from the ET modulator.